Systems approach to control early and late blight in potatoes

To prevent and control late blight (Phytophthora infestans) and early blight (Alternaria spp.) in organic potatoes, Aarhus University (AU) developed an existing early warning system for conventional potatoes to include organic potatoes. The system includes a disease surveillance system using smartphones and a GIS-based dashboard to display data, early genotyping of pathogen population to know the “enemy”, a decisions support system (DSS) to estimate the favorability of the weather for infection and thus recommend fungicide application only when necessary. The systems also includes monitoring of stability, type and level of resistance in the cultivars.

Initially, the Organic-PLUS potato group identified three scenarios for phasing out copper (Fig. 1). Although copper is not allowed in Denmark, these scenarios offers a robust approach towards achieving a strong organic potato sector. A prominent component in the three scenarios (Fig. 1) is the use of resistant cultivars. Host resistance can be an extremely effective means of controlling both late and early blight as it is ‘built-in’ to the crop for the duration of the season. Currently, there are a good number of resistant starch potato cultivars and new but more resistant waxy potato cultivars are on their way to the market, which will offer organic growers an effective means of managing late blight without relying on copper. Previous studies have shown that R-genes deployed in resistant cultivars, particularly against P. infestans, can be ephemeral, owing to the ability of P. infestans to rapidly evolve to break the R-genes. Protecting the R-genes via the application of products/compounds that can suppress infection is vital for prolonging the effectiveness of R-genes. Alternative products are available for organic growers and the use of a DSS can enable optimal use of these products. Modern DSSs make use of precision agriculture technologies as indicated for Scenario 3.

Figure 1. Scenarios for phasing out copper fungicides for potatoes

The potato group at the Swedish Agricultural University (SLU) and AU have tested a wide range of biological control agents (BCAs) with a direct effect on the pathogens, plant resistance inducers and bio-stimulants that strengthen plant vigor. Generally, these products provide effective control against late and early blight under controlled conditions (e.g.  greenhouse, lab) but not under field conditions.

A wide range of products (Fig. 2) were tested  against late blight under field conditions at AU, Flakkebjerg during the 2019 and 2020 growing seasons. Generally, the weather in 2019 was more favorable for late blight than 2020. We also carried out artificial inoculation in 2019 but not in 2020, thus creating a higher inoculum pressure in 2019 compared to 2020. In 2019, only the phosphite-based product (Resistim 0-7-11) had a significant effect on late blight control compared to untreated potatoes. Only 4 of 11 treatments obtained a mean control effect higher than untreated in 2019. In 2020, 8 of the treatments obtained a higher effect than the untreated control, Resistim 0-7-11 and Kumulus S, 25% higher AUDPC than the untreated control (Fig. 2). The key message from these experiments is that infection pressure is very important when assessing the effect of the alternative products. The efficacy (late blight control) of the products were generally higher under low infection pressure (2020) than under high infection pressure (2019) (Fig. 2).  The recommendation is to adopt sanitation measures (e.g. healthy and disease-free seed potatoes, crop rotation) to reduce local primary inoculum sources. In 2021, we will continue the test of alternatives as well as using our adapted DSS to time the application of those that are most promising. Better timing according to the risk of infection is supposed to increase the effect against disease development. The next step will be to test a combination of compounds with different modes of action and combine this with the knowledge of the host type and level of resistance.

Figure 2. Test of different alternative products to control late blight in a year with high infection pressure (2019) and a year with low infection pressure (2020).

As research is ongoing to identity effective products which control disease and are tenable in organic production, the need for an effective early warning system and DSS will be important ensure they are used correctly. We developed a smartphone App and its associated surveillance dashboard to identify both early blight and late blight outbreaks in their initial stages. Over 50 registered users from the extension, breeding, industry and research sectors upload data to the Dashboard and the farming community can follow the onset and severity of late and early blight as well the cultivars affected in different regions. This information is crucial in deciding when and how to start a preventive use of either fungicides or any biological product (Fig. 3). Attribute data and photos are available and as such, this tool is valuable as a learning tool as well. 

Figure 3. Late blight and early blight warning using the BlightTracker smartphone App and the associated dashboard.

For testing the DSS in field experiments, we developed a simple interactive user interface where the technical people running the trials have access to the trial plan, specific trials, selection of weather data to use, model output to display etc. Simple horizontal bars indicate for each day if the risk of infection is zero (green), low (yellow), medium (orange) or high (red). After a spray, the protection period is indicated for each treatment with a green bar. This approach was initially developed for use in conventional potatoes and with dynamic dosages of traditional fungicides, but it will be adapted and tested with alternatives that have different modes of action in 2021 in Denmark and in 2022 in both Denmark and Sweden. The DSS includes several model components for both late blight and early blight, aiming towards the integrated prevention and control of both diseases.  

Figure 4. User interface of late blight and early blight DSS adapted for test in experimental field trials.

Key messages

  • Host resistance is key for successful management of late and early blight.
  • The tested alternatives to copper are more effective under low disease and infection pressure.
  • We have developed a robust and integrated system for early warning and forecasting of the infection pressure of late and early blight.  
  • The new system is pivotal our quest to optimise the efficacy of alternatives to copper.

Jens G. Hansen & Isaac K. Abuley, Aarhus University

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